As a yoga instructor in New York City, Jennifer Parmelee knows what to do to find her inner calm when hit with daily stresses. Instead of feeling overwhelmed by pressures or annoyances like being stuck in the subway, Parmelee uses them to keep her motivated.

"You need stress to a certain degree," she says. "You just try not to let it take control of you," she says.

For her, the idea is to turn a stressful situation into "fierceness or fun." Stress ... fun? Could stress actually be good for you?

In small doses, yes.

We may talk about cutting the stress from our lives, but we need those precious, powerful fight-or-flight hormones our bodies produce when we're about to be hit by a car or when confronted with an unexpected, needed-it-yesterday deadline at work. When the brain perceives physical or psychological stress, it starts pumping the chemicals cortisol, epinephrine (adrenaline) and norepinephrine into the body. Instantly, the heart beats faster, blood pressure increases, senses sharpen, a rise in blood glucose invigorates us and we're ready to rock. Or leap away from the car.

"Stress is a burst of energy," says psychiatrist Dr. Lynne Tan of Montefiore Medical Center in New York City. "It’s our body telling us what we need to do."

Moderate amounts of stress — the kind of short-term buzz we get from a sudden burst of hormones — can help people perform tasks more efficiently and can improve memory. Good stress is the type of emotional challenge where a person feels in control and provides some sense of accomplishment. It can improve heart function and make the body resistant to infection, experts say. Far from being something we need to eliminate from our lives, good stress stimulates us.

"Think about your daily life — when do you get things done?" asks Janet DiPietro, a developmental psychologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore. "When you have a deadline, when you have to perform. You want some stress to help you do your best."

The upside of stress
Increasingly, researchers are probing the upside of stress. Some believe short-term boosts of it can strengthen the immune system and protect against some diseases of aging like Alzheimer's by keeping the brain cells working at peak capacity. People who experience moderate levels of stress before surgery have a better recovery than those with high or low levels, another study showed. Recently, a study suggested that stress could help prevent breast cancer because it suppresses the production of estrogen. And earlier this year, research out of Johns Hopkins found that children of mothers who had higher levels of the stress hormone cortisol during pregnancy were developmentally ahead of those of women with lower levels.

"Those powerful chemicals are there, first and foremost, to help you survive," says Monika Fleshner, a neuroimmunophysiologist at the University of Boulder, Colo. who has studied stress' effect on the immune system. "It’s only under the circumstances of chronic stress or extreme, severe stress that we suffer negative effects."

Of course, there's the rub. Stress can be positive, but get too much of it — when the flood of hormones bombards your body longer than 24 hours, doctors say — and all kinds of bad things start to happen. Long-term, chronic emotional stress that lasts weeks or months is blamed for high blood pressure, heart disease, exhaustion and depression.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

"Over time, if you're constantly in fight-or-flight, if your heart muscles and valves are awash in the epinephrine, it causes changes in the arteries and in the way that cells are able to regenerate," says Tan.

The problem is, it's difficult to shut off the onslaught of stress hormones when they become harmful. People can't control how high their hormones go when they experience a difficult situation.

The body does give off signals when healthy tension has tipped over into bad stress. Mental fogginess, frequent colds, increased sensitivity to aches and pains are all signs of an overwhelmed immune system. Autoimmune diseases like psoriasis, arthritis and inflammatory bowel disease often flare up.

"What we can do is change the way our brains respond to [stress] with coping techniques such as deep breathing, meditation and exercise," says Dr. Bruce Rabin, a professor of pathology and psychiatry at the University of Pittsburgh Medical Center.

Moderate exercise elevates stress hormones in the body and, through its cardiovascular benefits, actually makes the brain and body more resistant to psychological stress, he explains.

Matter of perception
Indeed, stress is a doubled-edged sword that affects everyone differently. It's mostly a matter of perception. A speeding ride on a roller coaster is torture for some, while others race for the next ride. Multi-tasking or living in a hectic urban environment is a thrill for some, a confusing sensory overload for others.

The goal isn't an absence of stress. It's an unavoidable reality. Besides, without it, life would be a pretty dull existence. The key is channeling stress energy into productive action instead of feeling overwhelmed, experts say.

"Focus the energy like a laser beam on what you need to do," says Tan. "Very successful people, rather than feeling disempowered, take the extra stress energy ... and make it into a high-energy, positive situation."

Dr. Paul J. Rosch, president of the American Institute of Stress compares stress to the tension in a violin string.

"Not enough produces a dull, raspy noise and too much results in an annoying shrill or snaps the string. However, just the right amount of stress creates pleasing sounds," he says.